Pyroclastic density currents are among the most dangerous hazards during explosive volcanic eruptions, typically having catastrophic and lasting impacts on society, infrastructure, and landscape evolution of the area. After the eruption of Unzen volcano, Japan, in 1991, during which 43 people were killed when pyroclastic surges unexpectedly separated from the parental flows, the possibility of decoupling in block‐and‐ash flows and the potential hazard of this was recognized. In the following years, decoupling has been documented at several composite volcanoes, but still not enough is known about the mechanics of pyroclastic currents, which allow the detachment of ash cloud surges. In this thesis, several processes thought to initiate decoupling in pyroclastic currents, such as entrainment of substrate at the flow base or of air at the flow front, elutriation of fines into the upper ash cloud surge or simple gravity segregation, are investigated. These mechanisms lead to increased non‐uniformity and stratification, which is a prerequisite for the onset of decoupling in small‐volume block‐and‐ash flows. Other mechanisms such as topographic control over block‐and‐ash flow dynamics are also considered, with examples confirming the importance of topographic influence for flow stratification and decoupling in block and‐ ash flows. Detailed field studies at Tarawera Volcano, New Zealand, have provided comprehensive descriptions of the distribution and sedimentology of block‐and‐ash flow deposits emplaced during the Kaharoa eruptive episode in AD 1314 ± 12, and these confirm the importance of changes in topography on flow dynamics. Topographic variations causes channeling, blocking and deceleration of the basal flow parts at Tarawera Volcano, while the upper flow parts are unconfined and decoupled, leading to detached ash cloud surge deposits beyond the limits of the main block‐and‐ash flow deposits. Interaction of the advancing flow with the substrate resulted in dynamic interaction. Deformation features and erosion gullies confirm the highly erosive nature of the flows. Laboratory‐scaled simulations of aqueous glycerol solutions and glass particulate currents are used as quantitative semi‐guides for pyroclastic flow behaviour, with special regard to decoupling caused by irregular topographies.